Field of the Invention
The present invention relates generally to poly phase filters for dot sequential color difference signal conversion. More particularly, the invention relates to a poly phase filter having simple hardware construction which is capable of filtering luminance Y and color difference signals RY/BY of a digital video signal.
Generally, the input conditions of an SD VCR are a luminance signal Y of 13.5 MHz and a color difference signal RY/BY of 13.5 MHz. In order to satisfy such input conditions of the SD VCR in the case where a digital video signal of 14.3 MHz (4 Fsc) is applied to the SD VCR, the digital video signal of 14.3 MHz is converted into an analog video signal of 14.3 MHz. The resulting 14.3 MHz analog signal is in turn, converted into an analog video signal of 13.5 MHz. The analog video signal of 13.5 MHz is again converted into a digital video signal of 13.5 MHz.
This multi-conversion method for converting the 14.3 MHz digital video signal into a 13.5 MHz digital video signal is disadvantageous because a signal loss results from each respective conversion step, resulting in inefficiency in the signal processing.
Alternatively, the digital video signal of 14.3 MHz may be converted directly into the digital video signal of 13.5 MHz according to a frequency sampling program, as shown in FIG. 1.
FIG. 1 is a schematic block diagram illustrating the construction of a conventional N-tap poly phase filter for conversion of digital signals. As shown in this drawing, the conventional N-tap poly phase filter comprises a delay group and a coefficient group. The delay group includes a plurality of delay units for sequentially delaying input data, and the coefficient group includes a plurality of coefficient units. Each coefficient unit outputs a phase filter coefficient to be multiplied by data output from a corresponding delay unit in the delay group. Each of the coefficient units includes k different poly phase filter coefficients which are periodically and repeatedly output from the coefficient units by a counter.
The conventional N-tap poly phase filter further comprises an adder for adding all the results obtained by multiplying 1) the output data from the delay units in the delay group by 2) the poly phase filter coefficients from corresponding coefficient units in the coefficient group. The output of the adder is provided as the final output of the poly phase filter in a manner similar to a general digital filter.
A point of distinction between the poly phase filter and a general digital filter is that coefficient variation is performed in a poly phase filter. The digital filter may perform an inaccurate operation in response to the input signal since the filter does not provide poly phase filter coefficient variation. The poly phase filter can perform the poly phase filter coefficient variation to make up for the deficiency of the digital filter and to ensure more accurate operation.
FIG. 2 is a schematic block diagram illustrating a dot sequential color difference signal conversion method using the conventional N-tap poly phase filter in FIG. 1, and FIGS. 3A to 3G are views illustrating the operation of FIG. 2. In order to convert luminance signal Y and color difference signal RY/BY of 14.3 MHz into a luminance signal Y and color difference signal RY/BY of 13.5 MHz, respectively, the 14.3 MHz luminance signal Y and color difference signal RY/BY are first separated from each other. The color difference signal RY/BY is then separated into color difference signal components RY and BY. Then, the luminance signal Y and color difference signal components RY and BY are filtered by corresponding poly phase filters.
In the case where a 7-tap poly phase filter is used, an input color difference signal RY/BY, as shown in FIG. 3A, is separated into a color difference signal component RY, as shown in FIG. 3B, and a color difference signal component BY as shown in FIG. 3E. The separated color difference signal component RY is filtered in a convolutional manner with poly phase filter coefficients such as that shown in FIG. 3C. The filtered result is shown in FIG. 3D.
On the other hand, the separated color difference signal component BY is filtered in a convolutional manner with poly phase filter coefficients as shown in FIG. 3F. The filtered result is shown in FIG. 3G.
The method for converting the 14.3 MHz digital video signal directly into a 13.5 MHz digital video signal according to the frequency sampling program of FIG. 1 has a disadvantage in that it requires a plurality of poly phase filters, one for each of the luminance signal Y and color difference signal components RY and BY. This requirement for multiple poly phase filters increases the hardware size of the converting apparatus and its associated cost.